1. Field of the Invention
This invention relates to a method of manufacturing heterojunction bipolar transistors.
2. Description of the Prior Art
With the recent progress of the epitaxial techniques for III-V group compound semiconductors, particularly AlGaAs compound semiconductors, heterojunction devices have already been made. An example of such a device is an AlGaAs/GaAs double heterojunction semiconductor laser which is manufactured by a liquid phase epitaxial growth (LPE) technique. There have been developed techniques for manufacturing the above-mentioned devices such as a vapor-phase growth method and molecular-beam epitaxy method which permit production of devices which require abruptness of a hetero interface. These devices include a high electron mobility transistor (HEMT) and a heterojunction bipolar transistor (HBT) according to the present invention.
The heterojunction bipolar transistor is a type which can overcome defects of a monojunction bipolar transistor made, e.g., of silicon or the like. To specifically explain the advantage of, for example, a heterojunction type transistor using AlGaAs for its emitter (E) and GaAs for its base (B) and collector (C), the hole or the majority carrier in the base cannot be diffused into the emitter due to an energy barrier produced by a band gap difference (.DELTA.Eg) between E and B, so that the base current is decreased and the injection efficiency of electrons from the emitter to the base in increased. It is therefore possible to increase the amplification degree (.beta.=I.sub.C /I.sub.B) even though the base concentration is set at a high value and the emitter concentration at a low value. This means that the base resistance and the capacitance between E and B, which relate to the operating speed of the device, can be decreased. It has been theoretically and experimentally shown that the heterojunction bipolar transistor can operate at a higher speed than a silicon bipolar transistor.
The structure of the heterojunction bipolar transistor can take the form of a mesa type as shown in FIG. 1 or a planar type as shown in FIG. 2.
Referring to FIG. 1, a mesa type bipolar transistor 1 is formed of a conductive layer made of n.sup.+ -GaAs with a thickness of 5000 .ANG., i.e., a collector electrode leading layer 3, a collector region 4 made of n-GaAs with a thickness of 3000 .ANG., a base region 5 made of p-GaAs with a thickness of 1000 .ANG., an emitter region 6 made of n-AlGaAs with a thickness ranging from 1000 to 1500 .ANG., and a cap layer 7 made of n.sup.+ -GaAs with a thickness ranging from 500 to 1000 .ANG. successively deposited on a semi-insulating GaAs substrate 2. Reference numeral 8 designates a base electrode, 9 an emitter electrode, 10 a collector electrode and 12 an external base layer.
The mesa type bipolar transistor 1 has a portion of the external base layer 12 and a part of the collector electrode leading layer 3 exposed by etching, on the surfaces of which the electrodes 8 and 10 are directly deposited. An isolation between elements is also carried out by etching to the depth of the substrate 2. Reference numeral 11 designates an insulating layer made, e.g. of SiO.sub.2 or the like. The mesa type transistor is advantageous in that a single element for experimental use can be manufactured in a shorter time period and with less processing. However, it provides some problems in practical manufacturing, e.g., as follows:
1. It is difficult to expose the external base layer by etching with good reproducibility;
2. When the isolation between elements is carried out, there is a possibility that wiring is cut at the edge of the stage in the case of a one-stage mesa structure. If a plurality of small stages is formed in place of one stage so as to prevent the cutting at edges, it entails an increase in device size.
3. There is the same problem as to the formation of an electrode on the collector electrode leading layer.
Therefore, the so-called planar type transistor, which has a plane surface to solve the above-mentioned problems inherent in the mesa type transistor, is an indispensable structure for practical semiconductor integrated circuits (IC).
FIG. 2 shows a typical structure of an AlGaAs/GaAs planar heterojunction bipolar transistor manufactured by ion implantation and metal burying technologies. An example of a manufacturing process of a transistor 17 having such a structure will be brfiefly explained as follows.
As shown in FIG. 2, on a semi-insulating GaAs substrate 2, there are epitaxially deposited successively an n.sup.+ -GaAs layer which is to serve as a collector electrode leading layer 3, an n-GaAs layer which is to serve as a collector region 4, a p-GaAs layer which is to serve as a base region 5, an n-AlGaAs layer which is to serve as an emitter region 6, and n-GaAs and n.sup.+ -GaAs layers which are to serve as a cap layer 7. Then, the cap layer 7 made of n.sup.+ -GaAs except the portion which will become the emitter region 6 is removed by etching. Next, Mg is implanted with a resist or an SiO.sub.2 layer 11 used as a mask, and thereafter an external base layer 12 is formed by annealing. Further, an element isolation region 13 and a base/collector isolation region 14 are formed by implanting B.sup.+ or H.sup.+ ions thereinto. Finally, a portion of the SiO.sub.2 layer 11 over the collector electrode forming layer is removed to form an opening, a trench (groove portion) 15 is formed, and a metal is buried into the trench 15, to thereby manufacture the transistor 17.
The planar transistor has excellent features in comparison with the mesa transistor as described above. However, conventional methods of manufacturing planar transistors still provide the following problems:
Since the external base region is formed by implanting Mg ions and annealing, the contact resistance and the sheet resistance of the external base region cannot be sufficiently reduced. Further, since the external base region is formed by implanting Mg ions, the emitter concentration of n-AlGaAs and GaAs cannot be made high, because then the emitter resistance becomes high. Further, there occurs a problem in controlling the minute emitter area due to diffusion of Mg. It is therefore difficult to obtain homogeneity in such devices, resulting in scattering of characteristics among such devices.